1. Field of the Invention
This invention relates to an improved method of preparing aromatic dicarboxylic acid chlorides by the reaction of aromatic dialdehydes with chlorine.
2. Description of the Prior Art
Aromatic dicarboxylic acid chlorides, e.g., terephthaloyl chloride, undergo polymer-forming condensation reactions with diamines and glycols and thus are important as intermediates in the preparation of polyamides and polyesters. One of the methods known to the art for preparing such acid chlorides involves the reaction of aromatic aldehydes with chlorine. In this method, which is described in U.S. Pat. No. 3,274,242, the chlorination is conducted in the vapor phase at elevated temperatures, e.g., in the range of about from 350.degree.C. to 450.degree.C. for terephthalaldehyde chlorination. The presence of the dialdehyde in the vapor phase together with the use of high temperatures heretofore has been indicated to be critical to obtaining a high yield of the dichloride in this reaction. Also, large excesses of chlorine have been indicated to be beneficial. However, despite such relatively severe conditions, the use of which is associated with economic and technical disadvantages, the vaporphase chlorination method has been shown to give only a relatively low degree of conversion of the aldehyde to the chloride, i.e., less than 40 percent.
The art also describes the chlorination of certain liquid aldehydes, i.e., aromatic monoaldehydes, at temperatures of 125.degree.C. to 160.degree.C. (Gilman, Organic Syntheses, Coll. Vol. 1, Ed. 2, 1941, p. 155), but the reaction has been shown to be discouragingly slow, i.e., on the order of 15-30 hours of o-chlorobenzaldehyde chlorination.